1. Technical Field
The present invention relates generally to secure communications when data must be sent from a transmit device to a receive device in an encrypted form and particularly refers to a method of dynamically updating encryption keys without having to transmit them.
2. Description of the Related Art
Secure communications systems, based on cryptography, are used to prevent unauthorized access to data on communications links so that sensitive information can be exchanged with little risk of eavesdropping. In typical point-to-point cryptographic systems, an encryption device transmits encrypted or coded digital data to a decryption device over a secure data link.
The digital data is encoded using a key known only to the encryption and decryption parties in order to deny data access by any unauthorized third party. This scheme implicitly refers to symmetric encryption, the conventional method of insuring security in the exchange of information being characterized in that the same key is used both for encryption and decryption. The security of symmetric encryption remains in the key i.e., divulging the key means that anyone could encrypt and decrypt messages. That is why the algorithms used for symmetric encryption are also referred to as secret-key algorithms. The best example of this is DES, which stands for Data Encryption Standard, and which has been indeed a standard since the 70's and is still universally used. Again, the security of such an algorithm resides only in the key and does not depend, whatsoever, on the secrecy of the algorithm. On the contrary, as a standard, the algorithm is completely specified and made available to all users.
DES is a block cipher algorithm wherein data are encrypted and decrypted in 64-bit blocks. A 64-bit block of text is converted by the algorithm into a 64-bit block of cipher text, with no overhead, except the necessary padding to make the whole message encode a multiple of 64 bits. The basic key is 56-bit long although it is expressed as a 64-bit number since a parity bit, per byte, is used for parity checking however, ignored by the algorithm. Thus, the key can be any 56-bit number and can possibly be changed any time provided both parties have a secure means to agree on a new key before exchanging data. To improve the strength of DES i.e., to increase the difficulty of breaking it, triple pass DES with 112-bit key or 168-bit key is also commonly used today. Because DES has been around for over 20 years, it has been thoroughly tested and has behaved remarkably well against years of cryptanalysis. Although it is still secure, it becomes obviously relatively weaker due to the dramatic increase in power computation now available in a single computer and even more in a group of computers cooperating to break such a code. Therefore, breaking DES consists ‘only’ in retrieving the particular key that was used to encode a message or a file since algorithm itself is completely known as stated above. The trivial way of achieving this being a brute-force attack in which all 256 keys of standard DES would be tried, insures that, on the average, after 228 attempts, encryption key may be found.
Thus, besides the length of the key, which is the prime contributor to prevent DES to be easily cracked, the other very important contributor is the duration during which such a key stays in use between two parties that want to keep secret the content of the information they are exchanging. As a general rule no encryption key should be used for an indefinite period of time since the longer a key is used, the greater the risk it will be compromised (loss or accident e.g., a key could be accidentally displayed in clear due an application software bug) and the greater the temptation for someone to spend the effort necessary to break it. Breaking a key shared by two banks for an extended period of time e.g. one month, would enable a hacker to interfere in the exchange of money between those two banks during the same period of time. Also, the more data are secured with a given key, the more devastating the loss if the key is compromised. Finally, a long lifetime of a key also provides more ammunition for an adversary to break it since the adversary potentially has access to significantly more data to work with. Thus, it is clear that it would be highly desirable that keys should remain in use for short or very short periods of time (and ideally only for one session) so that no attack could reasonably be conducted with a good chance of success before key is updated. However, this brings the usual problem, in symmetric cryptography, of having to distribute the keys shared by pairs of users over a large network.
To solve this problem, asymmetric encryption was devised. RSA algorithm (named after its creators, i.e., Rivest, Shamir and Adelman) and asymmetric algorithms in general solve the problem by using two keys: one private (secret) key and one public key. Public key of party A is made accessible to anybody who has to talk to A. Thus, when party B is ready to talk to A, it must use public key A to encode a message. Because the scheme is asymmetric, only A using its private (secret) key, will be able to decode the message. Therefore, the message can contain the secret key to be shared by two users utilizing the symmetric encryption method previously described i.e. DES. This method, which is a standard is, in practice, required because RSA and generally speaking all known public-key algorithms assume a lot of calculations involving exponentiation and/or discrete logarithms to be computed on very large numbers. A RSA secret-key is now commonly a 1024-bit binary word (rapid progresses have been reported in the recent years on the cracking of RSA-like keys forcing the use of very large keys so that 2048-bit and 4096-bit keys are considered in the implementation of the crypto-processors specialized to process RSA and DES algorithms for fast encryption) assuming that exponential calculations in that range are performed to encrypt a message. Thus, RSA algorithm is reported to be several orders of magnitude slower than DES, which explain why both are most often combined to implement a cryptographic system.
Before the encryption, the compression of data is very often used in data communications systems. The objective is twofold. Besides limiting the memory required to store a compressed file, the chief advantage is that less data have to be transmitted overall, thus saving bandwidth on expensive communications lines. Also, data compression can make cryptanalysis more difficult. Most often, to launch an attack, especially a brute-force attack, a cryptanalyst needs a small amount of cipher data and the corresponding plain data. In practice, this may not be difficult to obtain since communications protocols, at various levels, have standard message headers whose formats are well known.
Since data compression is performed on top of encryption, the corresponding plain data are meaningless or at least it becomes very difficult to match a particular protocol header. Especially, the numerous data compression techniques derived from the Ziv-Lempel method (J. Ziv and A. Lempel, ‘Compression of Individual Sequences via Variable-Rate Coding,’ IEEE Trans. Inform. Theory, vol. IT-24, no. 5, 1978) assume the use of an evolving dictionary in each node where data are compressed and decompressed. Then, dictionary contains the codeword representation based on a tree structure with brother, son and parent links and the corresponding character on each node or leaf. It is possible to start with an empty dictionary which needs to contain however, the first character of each sub-tree.
Although the dictionary is constantly evolving, it is kept identical on both ends while no specific data exchange need to take place to maintain the same contents. This is achieved from the transmitted data itself. With such a scheme, an identical evolving database is thus available on either end of a communication link while the changes cannot deduced, from the observation by an eavesdropper, of the data exchanged over the line. Therefore, a dynamic key can be derived from the directory contents using some form of one-way function such as hashing in order to frequently generate new keys.
Therefore, even if data compression before encryption can improve the security inasmuch as the compression increases the difficulty of cracking the encryption key, a perfect security can be obtained only by frequently changing the encryption key. Although RSA key can be used to exchange new secret keys, such an exchange involve resources, adds overhead and stops the normal data transmission since the security association should be restarted. This is why secret keys are not changed very often and would never be changed at each packet. But keeping the same key during some time opens ways to spy, modify, reroute or copy the data using copy and paste to another stream and is not safe whatever the complexity of the encryption is.